The present invention relates to a semiconductor device, and particularly to a semiconductor device having fuses for modifying electrical failures of the semiconductor device.
It is difficult to find electric failures or detective conditions generated in a manufacturing process of a semiconductor device (LSI) such as a memory during its manufacturing process. They become clear in an electric testing process executed after the manufacturing process. Therefore, the LSI is provided with at least one redundant fuse to modify the electric failures. The electric failures found out in the electric testing process can be modified by cutting and processing (fuse blow) the fuse with laser light.
The redundant fuse is formed, using a polysilicon layer, on a field insulating film formed on the surface of a semiconductor substrate and is covered with an insulating film as shown in, for example, a patent document 1 (Japanese Unexamined Patent Publication No. Hei 11(1999)-67054). Upon cutting of the fuse, it is cut by laser light together with the insulating film. Thus, a cut section of the fuse is exposed to the outside within a fuse aperture or opening during from after the cutting of the fuse to resin encapsulation.
In a scribing process executed during from after the cutting of the fuse to the resin encapsulation, the cut section of the fuse is subjected to charged water. When ESD (Electro Static Discharge) occurs in this state, there is a possibility that an electrical charge will pass from the charged water to the fuse cut section and a gate electrode of an internal transistor to thereby cause breakdown of a gate insulating film of the internal transistor. There is also a possibility that even by an electrical charge charged on a film with a chip mounted thereon, it will be discharged from the fuse cut section to the gate electrode of the internal transistor, thereby causing the breakdown of the gate insulating film.
A protection circuit that protects an internal circuit of a semiconductor device has been described in, for example, a patent document 2 (Japanese Patent Application No. 3526853). This internal circuit is connected to a ground potential terminal (high potential side) and a low voltage source terminal (low potential side) and operated at a negative potential. The protection circuit comprises a first diode connected between a ground voltage source terminal and an input terminal and operated in the backward direction where a negative polarity surge has occurred in the input terminal, a second diode connected between the input terminal and the low potential source terminal and operated in the forward direction where the negative polarity surge has occurred in the input terminal, an NMOS transistor connected between a substrate potential terminal and the low voltage source terminal, and first and second capacitors which divide the difference in potential between the substrate potential terminal and the low voltage source terminal and applies a source-to-gate voltage to the NMOS transistor. The respective capacitances of the first and second capacitors are selected in such a manner that the source-to-gate voltage of the NMOS transistor does not cause turning-on thereof over its threshold value in the normal case free of the electrostatic surge. Since a backward electrostatic surge current flows in the first diode where the negative polarity surge is intruded into the input terminal, a response time is taken and the electrostatic surge responds in the forward direction of the second diode, so that the electrostatic surge current flows from the second diode to the input terminal. The difference in potential between the substrate potential terminal and the low voltage source terminal becomes, approximately identical to the voltage of the electrostatic surge. Thus, the gate-to-source voltage of the NMOS transistor, which is divided by the first and second capacitors, exceeds the threshold value so that the NMOS transistor is turned ON. Therefore, the electrostatic surge current flows through the substrate potential terminal, the NMOS transistor and the second diode so that the internal circuit is protected from the electrostatic surge.
As described above, the redundant fuse of the semiconductor device is cut by laser and thereafter its cut section is exposed to the outside up to the resin encapsulation. Therefore, there is a fear that in the scribing process or the like, the gate insulating film of each transistor in the internal circuit will be broken due to the electrical charge intruded from the cut section. Although, however, measures against the ESD surge intruded from the input terminal such as described in the patent document 2 have heretofore been taken, any measures are not taken against the ESD surge at the cut section of the redundant fuse.